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Office of Columbia River

Definitions of Water Supply and Water Demand Used in the 2011 Forecast

Water Supply
Surface Water Supplies incorporate the impacts of operations of major reservoirs on the Columbia and Snake Rivers, as well as the major reservoirs in the Yakima.  Thus, with the exception of Yakima (WRIAs 37, 38, and 39), water supplies at the watershed (WRIA) level are “natural supplies”, without consideration for reservoirs.  Supplies reflect supply prior to accounting for demands.  They should not be compared to observed flows, which do account for demands through withdrawals for irrigation and other out-of-stream uses.

Groundwater Supplies were not modeled for the 2011 Forecast due to time and resource constraints.  Addressing this limitation will be a major focus of the 2016 Forecast.

Historical Supplies indicate surface water supplies for 1977-2006. This time period was selected based on the available data as the most appropriate comparison point for the future period.

2030 Forecast Supplies indicate forecasted supplies for the 2030s decade.  Major reservoir operations are assumed not to change in response to changes in forecasted 2030 water supply.  While this assumption may not be realistic, it was impractical to predict what management changes might occur.

Water Demand

Water Demands are derived under the baseline economic scenario unless otherwise noted. The baseline is defined to include medium domestic economic growth, medium growth in international trade, and no changes in water pricing or water supply capacity.

Agricultural Water Demand represents demand for water as applied to crops, often referred to as “top of crop”.  This includes water that will be used consumptively by crops, as well as water resulting from irrigation application inefficiencies (such as evaporation, drift from sprinklers, or runoff from fields).  In comparing these demands to supplies, it is important to include additional water to account for conveyance losses, such as occurs when transporting diverted water in unlined channels.

This is a physical, rather than an economic definition, where the latter would reference the quantity demanded at specific prices.  Agricultural water demand is forecasted under a projected crop mix that takes into account changes in domestic economic growth and growth in international trade.  The land base in agriculture is assumed to be the same.  The forecast does not incorporate improvements in irrigation efficiency or changes in crop mix that might be adopted by producers in response to limitations in water availability.

Water that is not consumptively used by crops (including irrigation application inefficiencies and conveyance losses) percolates through the soil and returns to the groundwater or surface water system.  Non-consumptive return flows may be available to users downstream although the time-lags vary considerably both in time and location.  Thus some of the upstream water demand will be counted towards supply downstream of the original place of use.

Conveyance Losses indicate water that is lost as it travels through conveyance systems (which can range from unlined ditches to fully covered pipes).  These losses vary widely and are difficult to assess, but have been estimated to average about 20% basin-wide.  Because of increased uncertainty associated with these estimates, conveyance losses have been treated and shown separately from “top of crop” demands.

Municipal Demand includes estimates of water delivered through municipal systems, as well as water delivered through self-supplied domestic systems. For those municipalities where data allowed, it also includes municipally-supplied industrial water.  It does not include self-supplied industrial water use. Municipal demand also has a consumptive portion and a non-consumptive portion, which includes water that is lost within the municipal system through system leakages and water that returns for wastewater treatment.  Together, the consumptive and the non-consumptive portion represent municipal diversion demand.

Instream Water Demand was incorporated into water management modeling through state and federal instream flow targets.   Within WRIAs, the highest adopted state and federal instream flows for a given month were used to express current minimum flows for fish in both historical and 2030 forecasted instream demands.  State and federal instream flows along the mainstem were also compared to historical and future supplies.

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